1. Field of the Invention
The present invention relates to an image data compression device and a compressing method capable of processing two types (natural images such as movies etc. and CG images such as a digital map etc.) of high quality images having different characteristics in real time, and a recording medium for performing the compression, and an image data decompression device and a decompressing method for decompressing compressed image data, and a recording medium for decompressing compressed image data.
2. Description of the Related Art
It is necessary to compress data to transmit image information containing an large amount of information. Especially, since 30 through 60 frames of image data are transmitted and received for moving pictures, the data compression is required.
It is well known that image information can be natural images such as general television images, movies, etc. and CG images (CG images) represented by maps for car navigation etc. Generally, natural images contain a number of low-frequency components, and digital images contain a number of high-frequency components. In a recent mobile terminal such as a car-mounted terminal, a mobile telephone, etc., both digital images such as a map etc. and natural images such as TV and movie pictures etc. are processed. To efficiently transmit both types of image data, an effective data compressing system for both low-frequency components and high-frequency components is desired.
The first conventional technology of compressing data using a JPEG (joint photographic experts group) and MPEG (moving picture experts group) shown in FIG. 1 is known as a common conventional image data compressing system. The following patent documents 1 and 2 belong to the first conventional technology. As shown in FIG. 1, the first conventional technology divides image data 110 into blocks (normally 8*8 pixels), performs frequency conversion 210 on the block image, performs quantization 310 on a DCT coefficient, performs variable-length encoding 410 for assigning a code depending on the frequency of occurrence, and transmits the result. The frequency conversion is to convert the frequency of image data normally using a DCT (discrete cosine transfer). Since human eyes are sensitive to low-frequency components (flat portion in an image), the DCT coefficient for a low frequency is precisely quantized, and the DCT coefficient for a high frequency is roughly quantized, thereby compressing natural images at a high compression rate at which image degradation can be inconspicuous. However, there is no problem with the compression of natural images when conspicuous low-frequency components are precisely quantized, but the image degradation is conspicuous on the high-frequency components such as a line and a character in map images (CG images). Furthermore, since edge information extraction 230 is performed on a block to be compressed, there is the problem that correction and feedback cannot be performed by detecting the image degradation caused by a quantization error. Therefore, it is not appropriate for CG images.
In addition, the second conventional technology using the JPEG-LS (lossless) as shown in FIG. 2 is known as a conventional image data compressing system. The second conventional technology predicts a level value using a MED (median edge detector) prediction device (the prediction device is a type of MAP (median adaptive predictor) from a level value of a pixel to be compressed when the image data 120 is compressed as sown in FIG. 2, and directly encodes prediction error. In this second conventional technology, since the frequency of occurrence of the prediction error basically concentrates on the vicinity of 0, the prediction error in the vicinity of 0 is assigned a short code while a prediction error having a large value is assigned a long code for compression. Although the compression rate is about 1/3, the encoding process is performed in a pixel unit regardless of high or low frequencies. Accordingly, no image degradation occurs even on CG images.
However, it is difficult to predict an interlace image and an image having low correlation between lines, and prediction error normally increases on these images. As a result, a variable-length code having a low frequency of occurrence (long code length) is assigned, and it is necessary to perform calculating process during encoding. Thus, each process becomes heavy and is not appropriate for real-time compressing process.
The prediction error is conspicuous on CG images when transfer or coding is performed by DCT such as the JPEG, MPEG, etc. while the prediction error on an interlace image and a landscape image is troublesome. Therefore, a long code is frequently used, and reduces compression efficiency.
Thus, a car-mounted image data compression device for processing both natural images and CG images has to compress CG images with high quality, and maintain the compression efficiency for interlace images and landscape images in movies and natural images.
Patent Document 1: Japanese Published Patent Application No. 2001-061149
Patent Document 2: Japanese Published Patent Application No. 2000-069479